Technical Field
Embodiments of the present invention relate to a sheet thickness detector, a sheet conveyor incorporating the sheet thickness detector, and an image forming apparatus incorporating the sheet thickness detector therein.
Related Art
Known image forming apparatuses generally form an image on a surface of a sheet during a sheet conveying operation. To obtain a high-quality image, image forming conditions are optimized according to different thicknesses of various types of sheets. For example, in a transfer process to transfer a toner image onto the sheet, a variety of sheet thicknesses has different volume resistance values in a transfer area. Due to the differences of the volume resistance values, an optimal transfer current value varies. Therefore, it is preferable to change the transfer current value according to the sheet thickness. Further, in a fixing process to fix the toner image transferred onto the surface of the sheet to the sheet by application of heat and pressure, different amounts of heat are applied to respective sheet thicknesses. Therefore, it is preferable to change a fixing temperature according to the sheet thickness.
Japanese Patent Application Publication No. JP 2012-166913-A discloses an image forming apparatus that includes a sheet thickness detector disposed in a sheet conveying path to detect a thickness of a sheet to be conveyed.
In Japanese Patent Application Publication No. JP 2012-166913-A, when a sheet enters between drive rollers and driven rollers, the driven rollers are pushed up due to the thickness of the sheet or sheet thickness. Accordingly, the position of a driven roller shaft is displaced to a direction to separate from the drive roller shaft. This amount of displacement of the driven roller shaft is detected by a displacement sensor that is disposed in contact with the driven roller shaft. According to the detection result obtained by the displacement sensor, a controller determines the sheet thickness.
However, due to nonuniformity in diameter of the drive rollers and the driven rollers, a gap is generated between the drive rollers and the driven rollers. The gap can cause variation in displacement amounts of the driven roller shaft. Consequently, a highly accurate sheet thickness detection cannot be performed. To detect and determine different sheet thicknesses accurately, a significant increase in cost for reducing allowable tolerance of a diameter of a roller is unavoidable.